FIG. 1 illustrates the current and power characteristics of a photovoltaic (PV) panel as a function of output voltage. The upper curve illustrates how the output current changes as the output voltage increases. Beginning at the far left side of the curve where the voltage is zero (short-circuit voltage) the output current remains relatively constant until the voltage reaches a point at which the current begins to curve downward. The current then falls off sharply and reaches zero at the open-circuit voltage VOC.
The lower curve is obtained by multiplying the corresponding current by the operating voltage to obtain the effective power at every voltage level. Beginning at the far left side of the curve where the voltage is zero, the power is also zero but increases until reaching a maximum value at VMPP. The power then decreases until reaching zero where the current falls to zero.
Referring to the top curve, the region to the left of the maximum power point (VMPP) is generally referred to as the current source region because output of the PV panel is generally a constant current. The region to the right of the maximum power point is generally referred to as the voltage source region because output of the PV panel is a relatively constant voltage.
Control of power converter and algorithms for maximum power point tracking (MPPT) often struggle to accommodate the transition between operating in the current source region and the voltage source region because transitioning between the two regions may change the dynamics of power converter control and the MPPT algorithm.